They get people linking to their page instead of the underlying YouTube video, hence better pagerank.
They get people visiting their page instead of just YouTube, and some of them reading other bits of their blog, hence more visibility for other stuff they want people to read.
They get their brand associated with Bill Gates doing cool philanthropic stuff.
The more difficult question is what benefit Hacker News gets from a link that goes there rather than to the underlying YouTube video. Actually, that's not a difficult question either; the answer is "no benefit".